This introduction by islaam.com
is intended to briefly describe envy as a disease of the heart, preceding
the text below it by Ibn Taymiyyah.

1. Envy is one of the major sins which is bound to destroy good deeds as
fast as the fire burns the wood and dry grass to ashes. “Jealousy eats away
at good deeds, just as fire eats away at firewood.” [Sunan Ibn Majah]
2. Envy was described as a sickness by the Prophet, sallallahu `alaihi wa
sallam.
3. Envy is hating that good happens to others.
4. Envy is displeasure with Allah's decree and His granting blessings upon
others.
5. "Faith and envy do not go together in the heart of a servant." [Ibn
Hibban, saheeh]
6. "The people will be fine as long as they do not envy one another." [At-Tabarani
with trustworthy narrators]
7. Envy is a characteristic the Jews displayed towards the Prophet,
sallallahu `alaihi wa sallam, causing them to inflict him harm.
8. Envy leads to hatred.
9. The Prophet, sallallahu `alahi w sallam, described envy as a shearer of
the religion.
10.Muslims are commanded in Soorah al-Falaq to seek refuge from the evil of
the envier when he envies.
11. Envy is worse than miserliness; this is because the miser only stops
himself from having good but the envier dislikes the favours of Allah
bestowed upon His servants.
12.No one attains true belief until one loves for one’s brother what one
loves for oneself.
13.It is said that the people who have the greatest degree of restlessness
are the envious. Such a person has no peace and is continuously grieved.
14. The greatest harm from envy comes to the envier, who with one’s
displeasure with Allah’s decree attains a great loss to oneself.

Al-Mubarrad recited the following lines:

The eye of the envier always sees scandal,
bringing out faults and hiding the good.

He meets you cheerfully, with a smiling face,
while his heart conceals his true feelings.

The envier’s enmity comes without provocation,
yet he accepts no excuses while he attacks.

Strictly speaking, envy (hasad) is hatred and disliking the good condition
of the envied one. This is of two types:

1) The blameworthy type of envy is unrestricted dislike of the blessings
bestowed upon the envied. This is the type of jealousy that incurs blame, so
when one hates something he is then hurt and grieved by the existence of
what he hates, and this becomes a sickness in his heart such that he takes
pleasure in the removal of the blessings from the envied even if this does
not result in any benefit to him except the single benefit of having the
pain that was in his soul removed. But this pain is not removed except as a
result of his continuously watching the envied so that the jealous person
finds relief when the blessing is removed, but then it becomes more severe
as is the case of the one who is sick, for it is possible that this
blessing, or one similar to it, returns to the envied. This is why the
second group said: 'It is a desire to have the blessings removed,' for
indeed the one who dislikes the blessings bestowed upon other than him
desires them to see removed.

2) That he dislikes the superiority of that person over him, and he desires
to be like him or better, so this is jealousy and has been called ghubta,
and the Prophet, sallallahu `alaihi wa sallam, called it hasad in the
hadeeth reported by both al-Bukhari and Muslim from the hadeeth of ibn
Mas`ood and ibn `Umar, radiyallahu `anhumaa, that he, sallallahu `alaihi wa
sallam, said, "There is no envy except in two cases: a person to whom Allah
has granted wisdom, and he rules by this and teaches it to the people, and a
person to whom Allah has granted wealth and property along with this the
power to spend it in the cause of Truth."

This being the wording of Ibn Mas`ood. The wording of Ibn `Umar is, "A
person to whom Allah has given the Qur'an and he recites it night and day,
and a person to whom Allah has granted wealth and property from which he
gives in charity night and day."

...So the Prophet, sallallahu `alaihi wa sallam, forbade hasad, with the
exception of two cases which are referred to as al-ghubta, meaning that a
person love the condition of someone else and dislikes that this person be
superior in this way (without his wishing that it be removed from that
person).

So if it is asked: 'Then why is this (ghubta) called envy when he loves only
that Allah bestow these blessings upon him?' It is said, 'The starting point
of this love is his looking towards the favors Allah has bestowed upon
someone else and his disliking that this person be favored over him. So if
this other person were not present then he would not have desired these
blessings. So because the starting point of this love is this dislike that
someone else be made superior to him, then this is called envy due to the
love following the dislike. As for desiring that Allah bestows favors upon
him without consideration of people's material conditions then this is not
envy at all.'

This is why the generality of mankind have been tried with this second type
of envy that has also been called al-munaafasah (competition) because two
people compete in a single desired matter, both of them trying to attain the
same good. The reason for their trying to attain it is that one of them
dislikes that the other be blessed with this matter over him just as any one
of two competitors dislikes that the other beat him.

Competition is not considered blameworthy in general, rather it is
considered to be praiseworthy when competing for righteousness. The Exalted
said,

"Indeed the pious will be in delight.
On thrones, looking on. You will recognize in their faces the brightness of
delight. They will be given to drink pure sealed wine. The last thereof
(that wine) will be the smell of Musk, and for this let those compete who
want to compete." [Al-Mutaffifeen (83):22-26]

So one is commanded to compete for these delights and not compete for the
delight of this fleeting world.

...The souls do not envy the one who is in severe hardship and this is why
the Prophet, sallallahu `alaihi wa sallam, did not mention it even though
the mujaahid, fighting in the Way of Allah, is superior to the one who is
spending wealth.... Similarly, the Prophet, sallallahu `alaihi wa sallam,
did not mention the one who prays, fasts and performs the pilgrimage,
because there is no tangible benefit attained from the people for these
actions by which the person can be exalted or disgraced, as can be attained
in teaching and spending.

Fundamentally, envy occurs when someone else attains power and authority;
otherwise the one who is performing these actions is not normally envied,
even if this person be blessed with far more food, drink and wives than
others, as opposed to these two blessings of power and authority, for they
cause a great deal of envy.

Allah praised the Ansaar with His saying,
"And they have no jealously in their
breasts for that which they have been given (the muhaajiroon), and give them
preference over themselves even though they were in need of that." [Al-Hashr
(59):9]

As for the jealousy that is totally blameworthy then Allah has said with
regards to the Jews,
"Many of the People of the Book wish
that if they could turn you away as disbelievers after you have believed,
out of envy from their own selves even after the truth has become clear to
them." [Al-Baqarah (2):109]
'They wish' meaning that they hope to make you aspostisise from your
religion out of jealousy. So jealousy was the deciding factor behind their
wish even after the Truth had been made clear to them. This because when
they saw you attain what you attained of blessings - in fact they saw you
attain that which they themselves had never attained - they became jealous
of you. Similarly this is mentioned in another verse, "Or
do they envy men for what Allah has given them of His bounty? Then We have
already given the family of Abraham the Book of Wisdom, and conferred upon
them a great kingdom. Of them were (some) who believed in him (Muhammad) and
of them were some who averted their faces from him and enough is Hell for
burning (them)..." [An-Nisaa' (4): 54-55]

"Say: I seek refuge with the Lord of
the Daybreak. From the evil of what He has created. And from the evil of the
darkening (night) as it comes with its darkness. And from the evil of the
witchcrafts when they blow in the knots. And from the evil of the envier
when he envies." [Al-Falaq (113):1-5]

...So the one who is jealous, hating the favours bestowed by Allah upon
someone else is an oppressor, going beyond bounds due to this. As for the
one who dislikes that someone else be blessed and wishes to be blessed in
the same way, then this is forbidden for him except in that which will bring
him closer to Allah. So if he were to wish for something that has been given
to someone else which would help bring him closer to Allah then there is no
problem in this. However, his wishing for it in his heart, without looking
to the condition of someone else is better and more excellent.

Then if this person were to act, dictated by this jealousy, he would be an
oppressor going beyond bounds, deserving of punishment unless he repents...

Jealousy is one of the sicknesses of the soul, and it is an illness that
afflicts the generality of mankind and only a few are secure from it. This
is why it is said, "The body is never free from jealousy, but debasement
brings it out, and nobility hides it." It was said to Al-Hasan Al-Basree,
"Can a believer be envied?" He replied, "What has made you forget Yoosuf and
his brothers, have you no father? But you should keep (this envy should it
occur) blinded in your heart, for you cannot be harmed by that which you did
not act upon in speech or action."

So the one who finds that he harbours jealousy in his soul towards someone
else, then it is upon him to treat it with patience and taqwaa of Allah, and
dislike it being in his soul... As for the one who actually takes a stance
against the envied, either with words or actions then he will be punished
for this, and the one who fears Allah and is patient and does not become one
of the oppressors - Allah will benefit him for his taqwa.

...In the hadeeth there occurs, "There are three sins from which no one can
be saved: jealousy, suspicion and omens. Shall I tell you of what will
remove you from this: When you envy do not hate, when you are suspicious
then do not actualise your suspicions, and when you see omens then ignore
them." Reported by Ibn Abi Ad-Dunya from the hadeeth of Abu Hurayrah.

In the Sunan from the Prophet, sallallahu `alaihi wa sallam, "You have been
afflicted with the illness of the nations that came before you - jealousy
and hatred. They are the shearers, I do not mean the shearers of the hair,
rather they are the shearers of the religion." [At-Tirmidhi, at-Tabaranee
and al-Hakim who said it was saheeh].

So he called jealousy an illness just as he called miserliness an illness in
his saying, "And what illness is worse than miserliness." [Ahmad, Hakim and
others, saheeh]

...In the first hadeeth jealousy was mentioned along with hatred. This is
because the envier, first of all dislikes the bounty bestowed by Allah upon
the one who is envied, and then begins hating this person. This is because
the hatred of the thing being bestowed leads to hatred of the one upon whom
it is bestowed, for when the blessings of Allah are bestowed upon an
individual, he would love that they go away, and they would not go away
except by the one who is envied going away, therefore he hates him and loves
that he not be there.

Jealousy necessarily leads to desire and hatred just as Allah informed us of
those that came before us that they differed,
"After there came to them knowledge out
of mutual hatred and desire." [Aal `Imraan (3):19]

“Do not envy one another, do note hate each other, do not oppose each
other…” [Al-Bukhari and Muslim]

“By the One in Whose Hands is my soul, none of you believes until he loves
for his brother what he loves for himself.” [Al-Bukhari and Muslim]

Greed is a sickness as is miserliness, and jealousy is worse than
miserliness, as occurs in the hadeeth, “Jealousy eats away at good deeds,
just as fire eats away at firewood.” [Sunan Ibn Majah]

This is because the miser only stops himself from having good but the envier
dislikes the favours of Allah bestowed upon His servants.